Power factor correction (PFC), is used to eliminate or at least to reduce harmonic currents in an input current. Harmonic currents can occur particularly in the case of nonlinear loads, such as, for example, rectifiers with subsequent smoothing in power supply units, since in the case of such loads, despite the sinusoidal input voltage, the input current is phase-shifted and distorted non-sinusoidally. The higher-frequency harmonics that occur in this case can be counteracted by an active or clocked power factor correction circuit connected upstream of the respective device.
Power factor correction circuits are also used in operating devices for illuminants, for example in electronic ballasts for fluorescent illuminants or in LED converters. The use of such circuits in devices for operating illuminants is expedient since standards restrict the permissible return of harmonics into the supply system.
A circuit topology based on a boost converter, also referred to as step-up converter or up-converter, is often used for power factor correction circuits. In this case, an inductance or coil supplied with a rectified AC voltage is charged with an input current or discharged by a controllable switch being switched on or being switched off. The discharge current of the inductance flows via a diode to the output of the converter, said output being coupled to an output capacitance, such that a DC voltage increased relative to the input voltage can be tapped off at the output. Other types of converter can likewise be used.
Power factor correction circuits can be operated in different operating modes. In particular, operation with a continuous current through the abovementioned inductance (so-called “Continuous Conduction Mode”, CCM), operation with a discontinuous inductance current or coil current (“Discontinuous Conduction Mode”, DCM) or operation in the borderline or boundary range between continuous and discontinuous current through the inductance (“Borderline Conduction Mode” or “Boundary Conduction Mode”, BCM) is known. In BCM operation a decrease in the coil current to zero during the discharge phase of the coil can be taken as a reason to start a new switching cycle and to switch the switch on again in order to charge the coil anew. The power factor correction circuit can be controlled or regulated by means of the time duration during which the switch is switched on in each case. In DCM operation, by contrast, after a zero crossing of the coil current during the discharge phase firstly there is a wait during a predefined additional waiting time until the switch is closed anew.
DE 10 2004 025 597 A1 describes a power factor correction circuit in which an output DC voltage is derived during the switched-off time duration of the switch.
When a power factor correction circuit is operated in the DCM mode, the waiting time before renewed switching-on of the converter can be chosen depending on a load, i.e. depending on an output power of the power factor correction circuit, in order to maintain a predefined bus voltage. If the switch is switched on again directly after this time has elapsed, this can lead to irregularities in the coil current. If the switch-on instant is chosen depending only on the predefined waiting time and independently of the behavior of the power factor correction circuit, an increased dissipation and thus heating of the switching means can also occur.
It is an object to specify methods and devices which offer improvements with regard to the problems mentioned. It is an object to specify methods and devices for power factor correction in which operation over a larger range of loads is possible. It is also an object to specify methods and devices in which the dynamic behavior of the power factor correction circuit during the period in which the switch is in the off state can be taken into account when determining the switch-on instant.